1. Field of the Invention
The present invention relates to a color filter layer for liquid crystal display device and more particularly, to a color filter formed self-aligned to stripes of electrodes of the display device.
2. Discussion of the Background
Liquid crystal flat panel display devices are increasingly used due to their low consumption of electrical power and small size. Liquid crystal display devices are widely used in information displays as well as various consumer products. Recently, the performance of large liquid crystal display devices has approached that of the cathode ray tubes.
Liquid crystal display devices typically include at least a layer of liquid crystals placed between a pair of light transparent substrates provided with alignment films and transparent electrodes, a pair of polarizers disposed on outward surfaces of the substrates.
Although a sheet of glass plate has been mostly used as the transparent substrate, a film of plastic material is increasingly used as substrates in liquid crystal displays for hand-held devices, such as cellular phones and portable pocket books, for example. Typically, the plastic films used as substrates are relatively thin, having a thickness of from 0.1 to 0.3 mm and of light weight.
This trend is expanded, as the demand increases for color display of data, including images in hand-held displays, which are typically exemplified by reflection type liquid crystal displays with a low power consumption.
One disadvantage arising from the use of plastic materials as such substrates is the difficulty in forming color filters through precise patterning process steps due to a relatively large change in the dimension of plastic materiels with environmental conditions, such as temperature and humidity. This results in the extensive use of monochrome displays.
Several methods have been proposed for displaying color images by liquid crystal displays without color filters. However, since these methods have limited capabilities, such as the number of color tones and vividness of displayed colors, color filters are primarily used for color displays at present. These methods are especially demanding for liquid display devices having plastic substrates, and have not been carried out successfully, since these methods require a strict control of cell spacing, for example, which is rather difficult to attain for the plastic materials.
Various methods of color filter fabrication are disclosed which include dyeing, dispersing pigments, electrodepositing, electrolyzing (or disrupting) micelles, printing and other similar methods. In the dyeing, dispersing and printing methods, either red (R), green (G) or blue (B) finer is formed on a transparent substrate.
These R, G and B filters are formed such that each is distributed substantially uniform, which requires a strict positional control during the fabrication. In addition, these color filter patterns are required to be positioned so as to be connected further to the electrodes of peripheral driving circuitry.
The precision feasible for the pattern registration is generally determined by the kind of, and the size of the substrate, for example, and the process equipment used for the fabrication. For glass substrates, a precision on the order of one micron can typically be maintained.
For plastic substrates, however, changes in dimension with temperature and humidity are generally larger than those of glass substrates. Depending upon environmental conditions as well as heat history of plastic materials, the changes sometimes amount to 0.1%, which results in difficulties in attaining precise registration, as noted above.
To form color fillers on plastics substrates with a sufficient precision for fabricating liquid crystal device with satisfactory device characteristics, it desirable to use fabrication processes which do not require precise positional registration or to reduce such stringent processes as much as possible.
In the above-mentioned methods of color filter fabrication, such as electrode position and micelle electrolysis, color filters are formed through electrochemical processes on a pattern of transparent electrodes which are previously prepared on the substrate, and a dislocation (or misregistration) can therefore be minimized with respect to each other among color filters, as will be described hereinbelow.
In addition, layers of the color filters may also be rendered conductive through these methods, by containing, or being admixed with conducting materials. As disclosed in Japanese Laid-Open Patent Publication No. 6-34809, the thus formed conductive color filter layers may serve as the electrodes not only for the electrochemical processes but also for driving liquid crystal devices.
For transmission type liquid crystal displays, a pattern of the black filters or a black matrix (BM) is commonly formed so as to fill gap portions between the RGB filters to thereby be able to enhance the contrast and color purify of the display. However, it is noted that the black matrix is not used in reflection type displays, since the reflection type displays are typically formed to sufficiently reflect incident light back to an observer and to provide a display as clear as possible.
It is also noted that the black matrix not only blocks light which bleeds through color filters, but also assists to level the surface of the display which is defined by the face of color filters which are disposed facing to a liquid crystal layer. Although the required surface flatness is dependent on the type of liquid crystal displays, it is typically on the order of 0.1 micron for super twisted nematic (STN) type displays.
As mentioned above, the color filters may be formed through electrochemical processes to be conductive and serve as the electrodes not only for the electrochemical processes but also for driving display devices. Further, as noted above, the surface flatness is considerably important for the STN type displays. Therefore, it is not desirable to form a thick leveling layer on top of respective color filters especially in the case of STN displays, because of a potential drop induced in the liquid crystal layer by the thick layer. That is, it is desirable to form the leveling layers only in gap portions between color filters, and either a thin or no leveling layer is formed on top of the color filters.
To make this possible, as long as the device is fabricated on a glass substrate, patterning process steps can be employed, including conventional photolithography techniques for the leveling layer composed of photo-curing acrylic resin. However, these steps are difficult to apply to a display device with a plastic film substrate because of the difficulties mentioned in precise positioning for the plastic material.